Based on recent findings from our team and others, we confirm initial reports that prostate cancer (PC) survivors treated with androgen deprivation therapy (ADT) are significantly more likely to fall and to become frail than PC survivors who do not receive this treatment or men without cancer, placing nearly half of all PC survivors at higher risk of fall-related fractures, other life-threatening injuries, disability and death. Currently, there are no evidence-based fall prevention strategies that specifically target PC survivors on ADT. The purpose of the proposed study is to determine and compare the efficacy of two very distinct types of exercise, tai chi versus strength training, to prevent falls specifically linked to ADT for prostate cancer. We will examine additional endpoints of frailty and physical function that are also worsened with ADT and contribute to the risk of falls and future disability. We propose a 3-group, single-blind, parallel design, randomized controlled trial in PC survivors treated with ADT (N=360) comparing 1) tai chi to 2) strength training against 3) a placebo control group of seated stretching exercise. Men will participate in supervised study programs 3x/wk for 6 months and will be followed for 6 more months after formal training stops. The specific aims of this study are to determine and compare the efficacy of both tai chi and strength training to 1) reduce falls and 2) lessen frailty (shrinking, weakness, slowness, exhaustion, and inactivity) and dysfunction (mobility, balance, self-report function) in PC survivors on ADT, 3) Determine whether or not the benefits of each intervention last after structured training stops and 4) Explore the patterns and predictors of men who respond best to each intervention. The proposed study is innovative in its approach because it applies two well-characterized fall prevention exercise programs (e.g., tai chi and strength training) to a new population just recently identified at high risk of falls and for whom the causes of falls and effective countermeasures remain unclear. This study is the very first attempt at a direct comparison of two known exercise modalities, each with distinct training properties, to reduce falls, frailty and dysfunction in PC survivors. The proposed project responds to an urgent research agenda in cancer survivorship that calls for developing appropriately targeted, effective, and sustainable exercise programs for cancer survivors and to NCI and IOM priorities to expand cancer care to prevent disability in older survivors. This study is relevant to public health because up to 45% of the 2 million PCS alive in the U.S. are prescribed ADT. Men on ADT can survive 16 years or longer, lengthening the time they grow susceptible to falls, frailty and dysfunction from aging and cancer treatment, yet there are no evidence-based therapeutic treatment options for these men. This study addresses this significant barrier in clinical practice by proposing a safe, low- cost, and scalable exercise-based approach. If the aims are achieved, an effective solution to prevent falls and reduce frailty associated with ADT will be readily available to clinical and rehabilitative practice. Importantly, this study would address a NCI Cancer MoonshotSM goal to reduce cancer's debilitating side effects.